I also consider this upgrade matrix to be slightly overkill. As mentioned by jeff, upgrading within a given stable release (e.g. 17.01.n --> 17.01.m) should always be safe with keeping settings. So far 17.01.x --> 18.06 also seems to be fine in general. Upgrading by keeping configuration files older than that (15.05.x or older) can not be considered safe in a large number of cases (because of changed switch/ vlan configurations, containing dnsmasq to its dedicated system user, etc. while there are mitigations in place, there is a high risk for soft-bricking the device in practice).
Providing any assumptions for even older releases (12.09, 14.07, etc.) -or for upgrades between versions of already long-unsupported releases- feels rather challenging, more than 4 years after every security conscious person has forgotten about them and the details required for upgrading them. Obviously their settings are not safe to be kept for upgrading to current (17.01.x/ 18.06.x).
This matrix might also transport the semblance of being able to avoid these compatibility issues by upgrading in stages, e.g. coming from 12.09 that you'd just have to upgrade to 14.07 first, before installing 15.05.x and 17.01.x by keeping the configuration files and never touching them - this is not going to help. sysupgrades in general don't 'migrate' config files during the upgrade, while the file list might be pruned slightly, the files are usually kept verbatim (yes, there are some upgrade scripts in place, attempting a config migration for targetted issues, like e.g. making sure to create a dnsmasq system user, but we've already established that these aren't always working). If your configuration has been created by firmware versions older than 17.01.x, it's not generally safe to retain them (and trying to restore an old configuration tarball would be even more fatal, as the existing/ incomplete migration scripts won't run in that case at all), even if it might work in some cases (depending on the chosen configuration and the devices in question).
Currently there are no (major) issues to be expected with upgrading from 17.01.0 or later, but there are always exceptions (e.g. lantiq changing the name of the dsl device) - and there will continue to be such issues (e.g. migrating targets to the dsa switch driver architecture).
As the issue has come up a few times recently, I'd like to add that restoring a configuration tarball made on an older router on newer/ different hardware is not possible, as very crucial and basic configuration settings are hardware dependent (switch and vlan configuration, LED/ GPIO settings, WLAN config (device paths), etc.).